Conventionally, an electron tube which is used for detecting the presence or absence of a flame on the basis of ultraviolet rays emitted from the flame in a combustion furnace or the like has been known. The electron tube includes a sealed container in which predetermined gas is sealed and filled, an electrode supporting pin that penetrates through the sealed container, and two electrodes that are supported in parallel with each other by the electrode supporting pin within the sealed container. In the electron tube, when one electrode arranged to oppose the flame is irradiated with ultraviolet rays in a state where a predetermined voltage is applied across the electrodes through the electrode supporting pin, electrons are emitted from the one electrode due to the photoelectric effect and excited in succession one after another to cause an electron avalanche between the one electrode and the other electrode. Therefore, it is possible to detect the presence or absence of a flame by measuring a change in impedance between electrodes, a change in voltage between electrodes, and electric current flowing between electrodes. Various methods for detecting the presence or absence of a flame have been suggested.
In the related art, there has been suggested a flame sensor in which electric current flowing between electrodes is integrated and it is determined that a flame is present in a case where an integrated value is greater than or equal to a predetermined threshold value and a flame is absent in a case where the integrated value is less than the predetermined threshold value (for example, see PTL 1). However, the flame sensor is a product having a lifetime and needs to be replaced appropriately. For that reason, it is required to detect a deterioration tendency of the flame sensor.
In the technically related field, in an ozone concentration meter disclosed in PTL 2, an optical path of light which passes through a reaction cell and another optical path of light which does not pass through the reaction cell are switched by an optical chopper. Light passing through the reaction cell is assumed as measuring light, light not passing through the reaction cell is assumed as reference light, each quantity of light is detected by a light receiver, both quantities of light are subjected to signal processing and comparison and calculation processing by a measurement circuit and thus, an ozone concentration value is calculated. In this case, the ozone concentration meter copes with a temporal change of a lamp which emits ultraviolet rays, using reference light. As such, the ozone concentration meter is a technique in which reference light and measuring light are alternately measured even without removing a sensor to thereby detect a change in sensitivity of the sensor.